Distribution networks, for example cable television distribution networks, are well known. In such networks, analog television signals are carried to customer terminals (television receivers) from a central station or head end via a branched coaxial cable which includes bridger amplifiers, line extenders, and customer taps. Each television signal occupies a typically 6 to 8 MHz channel at a frequency from about 50 MHz to about 750 MHz or more. The upper frequency is limited by the bandwidth of the bridger amplifiers and line extenders and the attenuation of the coaxial cable, which as is well known increases with increasing frequency. Various ways have been proposed for communicating additional signals via a cable television distribution network, typically involving the supply of such signals via optical fibres to appropriate points in the coaxial cable system with delivery of the signals to the customer premises via the coaxial cable, referred to as the drop cable, positioned between the customer tap to the customer premises. There is also a need to accommodate signals in the upstream direction from the customer premises to the central station, so that the network can then serve for communicating arbitrary types of signals in both direction. Such signals can include for example television program selection and control signals, metering signals, voice signals, and data signals.
In the downstream direction from the central station to the terminals, the cable distribution network constitutes a point to multipoint network, on which signal transmission is relatively easy to provide. In the upstream direction from the terminals to the central station, this network is a multipoint-to-point network, on which signal transmission is much more difficult to provide. One reason for the difficulty is the problem of contention among different terminals simultaneously wishing to transmit signals to the central station. Various collision detection schemes have been proposed generally for multipoint-to-point communications networks; these are not particularly suited to the cable distribution network, especially in view of the large number of terminals which may exist in such a network.
In addition, there can be a wide variation in the bandwidth which is required by each terminal, at any particular time, for transmitting signals in the upstream direction. For example, these signals can be isochronous (constant bit rate) signals, such as voice, metering, and video signals, and/or asynchronous (variable bit rate) signals, such as computer data signals, keyboard operations, and television push-button control signals. Providing for efficient transmission of these different types of signals from a large number of terminals is difficult.
In the past, systems of this type have generally employed a conventional TDMA protocol for the upstream transmission. In such an arrangement, a station wishing to transmit is allocated a time slot by the central station and occupies this time slot throughout the length of the transmission. This arrangement is however less than ideal as some of the terminals will have more data to transmit than can be easily accommodated in a single time slot wherein other terminals may have so little data that the occupied time slot is underused.
The data that is to be transmitted from customer terminal is generally of an asynchronous nature and it has been suggested that an asynchronous transfer mode (ATM) protocol would be more effective in terms of efficiency of use of the available bandwidth. The introduction of ATM techniques to this particular application has however been limited by the lack of an efficient marshalling scheme. In particular, a customer terminal transmitting ATM cells will in general be allocated different time slots in each frame and will thus need to send a transmission request to the central station for every cell that is to be transmitted. The present techniques that are available for sending such requests consume a significant proportion of the available upstream bandwidth thus reducing the bandwidth available for carrying traffic and limiting the operational efficiency of the system.